Mitzvos must be performed for free; we may not charge money for performing a mitzvah. For example, the Mishnah (Bechoros 29a) writes that: one may not charge money to judge between disputants, since judging is a mitzvah; one may not charge money to testify, since testimony is a mitzvah; one may not charge money to mix the ingredients of mei chatas, the waters that purify people from tumas meis, nor to sprinkle the mei chatas on a person, since these actions are mitzvos. The Gemara (ibid.) adds that one may not charge money to teach Torah, since teaching Torah is a mitzvah. The Ramban (Toras haAdam, inyan haMeichush) adds that physicians may not charge money to heal, since healing and saving lives are mitzvos.

However, there are actions that some Rishonim classify as mitzvos while other Rishonim do not. For instance, Ramban writes that the tircha, exertion, that a doctor expends when traveling to his patient is not a mitzvah, and that doctors  may charge a fee for that exertion. Similarly, Ramban writes that chalitzah is not a mitzvah, and one may charge a fee for performing chalitzah. This is true both when the yavam and yevamah are suitable for each other (i.e. they are of similar age and interested in one another as people) and when they are unsuitable for one another (i.e. they are of vastly different ages or he is only interested in her money). The Ohr Zarua (Hilchos Chalitzah), however, rules that when the yavam and yevamah are unsuited to each other, the yavam may not collect even for the tircha of traveling to perform chalitzah. This indicates that chalitzah is considered a mitzvah, and that the tircha of traveling to perform a mitzvah is considered part of the mitzvah itself.
The Mishnah (106b) writes that Beis Din should advise against yibbum and for chalitzah when a yavam is unsuited to his yevamah. The Gemara (106a) writes that if the yavam demands money for following this advice, he is not entitled to collect. Some Rishonim (see Ramban, ibid.) maintain that this extortionist yavam may not collect his fee because he has a mitzvah to peform chalitzah, i.e. a mitzvah and perhaps even an obligation to listen to the court’s advice. However, only one who extorts for performing a mitzvah is enjoined from collecting; one who extorts for non-mitzvah services may indeed be entitled to collect his fee. Ramban, though, writes that even when the yavam is an extortionist, chalitzah is not obligatory. Ramban probably feels that under these circumstances, although it is unwise to ignore the court’s advice, and it is immoral to act counter to the court’s advice, nevertheless, the court’s advice is not binding. Hence, Ramban learns from this Gemara that anyone who extorts an excessive monetary commitment, even in exchange for non-mitzvah service, is not entitled to collect.
What lies behind the dispute about whether or not tircha is part of the mitzvah? The Ramban cites a Mishnah (Bechoros, ibid.) to prove that tircha is not part of the mitzvah: one is permitted to charge for bringing the ingredients
of mei chatas to the person who will mix them. Presumably, this indicates that one may also charge for bringing the  mixer to the ingredients, as only the actual act of mixing is a mitzvah. The Ohr Zarua might respond that there is a  fundamental distinction between bringing the ingredients to the mixer, and the mixer traveling to the ingredients: the  mixer’s travels and the mixer’s mixing are performed by the same person (the mixer) and therefore combine into one elongated mitzvah action, while transporting the ingredients and mixing them need not necessarily be performed by  one person, and hence are considered two separate actions.
This thesis is borne out by an intriguing formulation of the Meiri (Yevamos 106a). Whereas Ramban writes that one may not collect an excessive fee for providing a patient with critical medicine, the Meiri writes that one may collect such a fee. The Meiri lists three reasons for this, one of which is that the injunction against collecting excessive fees “was only said about one who must use his body in the mitzvah. A person is not obligated to expend his assets [for free] for others to save them and their property, lest he later need those assets [to save himself]...” Just as here the Meiri does not consider parting with one’s assets part of the mitzvah, he may not consider bringing the ingredients of mei chatas part of the mitzvah; only actions that implicate the body alone are part of the mitzvah, but every such action, even tircha, is part of the mitzvah.